The present invention relates to a conveyor having self-propelled carriers, for example, for use in motor vehicle production lines.
With motor vehicle production lines, a plurality of carriers having vehicle parts (articles to be transported) placed thereon need to be moved through a specified work zone with a constant distance maintained between the carriers. Accordingly, power-and-free trolley conveyors have heretofore been used.
However, the power-and-free trolley conveyor employs chains and therefore has the problem of giving off a great noise and being unable to drive the carriers at a high speed in transport zones or the like other than the work zone.
Accordingly, conveyors having a plurality of self-propelled carriers have been proposed to ensure a reduced noise and to drive the carriers at a higher speed.
The self-propelled carrier of the conventional conveyor has one drive wheel rollable on a rail and one electric motor for driving the wheel.
The conveyor therefore has the problem that when the rail has a gradient, the drive wheel slips at the gradient portion, making the carrier unable to run smoothly.
To overcome this problem, it has been proposed to provide a carrier lift between a high horizontal rail portion and a lower horizontal rail portion without giving the gradient to the rail, or to provide a chain conveyor for the gradient portion to move the carrier with the chain (see Unexamined Japanese Patent Publication SHO 52-97573). However, the lift or chain conveyor needed results in an increased equipment cost.
A conveyor is also proposed wherein a rack having upward teeth is provided at a rail gradient portion, and a pinion mounted on the same shaft as the drive wheel of a carrier is adapted to roll on the rack at the gradient portion (see Examined Japanese Patent Publication SHO 52-97575). However, this arrangement has a problem in respect of strength since a load acts on the pinion at the shaft end of the drive wheel. Further because the pinion as positioned on the rack is subjected to the load of the carrier, the pinion fails to mesh with the rack smoothly, while these members undergo marked abrasion.
Accordingly, we have proposed a conveyor comprising a plurality of self-propelled carriers adapted to run along a rail, the rail having a first drive wheel bearing surface facing upward, a rack provided at a gradient portion of the rail and formed with a plurality of downward teeth, and a friction roller bearing surface facing downward and positioned in the rear of the gradient portion, each of the carriers having a first drive wheel rollable on the first wheel bearing surface of the rail, a first electric motor for driving the first drive wheel, a gear meshable with the rack teeth of the rail, a friction roller positionable in pressing contact with the roller bearing surface from below, and a second electric motor for driving the gear and the friction roller (see U.S. patent Ser. No. 07/466793, EPC90101036.3).
At the rail portion of the conveyor which is not provided with the rack nor with the roller bearing surface, the first electric motor drives the first drive wheel on the wheel bearing surface to cause the carrier to travel at a high speed. In the section where the rail has the roller bearing surface, the second electric motor drives the friction roller in pressing contact with the roller bearing surface to cause the carrier to travel at a low speed. In the section where the rail has the rack, the second electric motor drives the gear in meshing engagement with the rack to cause the carrier to travel at a low speed without slipping. At the rail portion of transit from the portion having the roller bearing surface to the portion having the rack, a roller rack is disposed which has a plurality of rollers arranged along the rail to smoothly bring the gear into meshing engagement with rack teeth. Usually, the gear properly meshes with rollers due to the rotation of the rollers to ride onto the rack, whereas it is likely that the tooth of the gear rides on the top of the roller upon coming into contact therewith. The gear then fails to come into meshing engagement with the roller rack smoothly.
To overcome this problem, it appears useful to replace the roller rack by a plurality of forward ratchet pawls which are arranged along the rail and which are movable only forward to escape but meshable with the gear. Even if the tooth of the gear strikes on the top of the ratchet pawl, the ratchet pawl will escape forward, permitting the gear to smoothly mesh with ratchet pawls. Nevertheless, it is still likely that the gear will be forced forward owing to a reaction by the ratchet pawl, failing to smoothly mesh with the rack.